As a crucial element for separation of reactants and anion transport, anion-exchange membranes are widely used in various industrial fields, including water treatment, hydrometallurgy, chemical industry, chemical separation, and electrochemical conversion devices, including fuel cells and electrolysis cells.
Fuel cells are devices that convert the chemical energy stored in a fuel directly into electricity and could potentially serve as a highly efficient and environmentally sustainable power generation technology for stationary and mobile applications. Fuel cells are promising energy conversion devices; however, improving their performance and enhancing durability remain significant challenges. Increasing the ionic conductivity and mechanical stability of solid polymer electrolyte membranes to achieve higher operating efficiencies are important goals. Within a fuel cell, the polymer electrolyte membrane acts as a barrier between the fuel and oxidant streams and simultaneously serves as the ion conducting medium between the anode and cathode. As a result, the membrane is a central, and often performance-limiting component of the fuel cell. Many low temperature (e.g., less than 100° C.) fuel cells employ a proton exchange membrane (PEM) as the electrolyte. The most common polymer electrolyte membrane fuel cells operate under acidic conditions and are therefore proton conducting. Nafion®, a PEM, has dominated the field due its processability, chemical and thermal stability, and proton conductivity (when properly hydrated). However, the use of these membranes is limited to acidic conditions and requires substantial dilution of carbon-based fuels (e.g., methanol) along with thicker (less efficient and more costly) membranes to prevent uncontrollable membrane swelling and fuel crossover. Additionally, PEM fuel cells rely almost exclusively on platinum, a very expensive and scarce noble metal.
A significant advantage of alkaline fuel cells (AFCs) over their acidic counterparts is greatly improved oxygen reduction kinetics as well as better fuel oxidation kinetics. These improvements can lead to higher efficiencies and enable the use of non-precious metal catalysts, greatly reducing the cost of the device. Indeed, hydrogen fueled AFCs can outperform all known low temperature (e.g., less than 200° C.) fuel cells. However, AFCs have traditionally employed liquid alkaline electrolytes containing metal hydroxides (e.g., potassium hydroxide) that react with CO2 (present in oxidant stream or fuel oxidation product when using carbon-based fuels) to form metal bicarbonates and subsequently carbonate salts. If sufficiently high levels of these salts are formed, they can precipitate out of solution, decreasing electrolyte conductivity and eventually obstructing electrode pores, both of which compromise power output.
There exists an ongoing and unmet need for conductive and solvent processable ionomers, which can be used as alkaline anion exchange membranes (AAEMs).